This invention relates to a method of continuously casting steel and more particularly to an improved method of continuously casting steel in which the position of the leading end of a crater in the cast piece is maintained at a definite position so as to prevent formation of center porosity at the leading end of the crater, a pipe or segregation and the like defect, thereby producing cast pieces of improved quality by improving the solidifed structure of the metal.
In the method of continuous casting of molten steel it has been the practice to adjust the draw out speed and the quantity of the cooling water such that the leading end of the not yet solidified crater in the cast piece is positioned at the pinch rollers. If the leading end of the not yet solidified crater advances beyond the pinch rolls, the cast piece would not be sufficiently pressed by the pinch rollers with the result that the solidified shell would be bulged outwardly by the static pressure of the molten steel in the crater. As a consequence, it would be impossible to form cast pieces of the desired dimension. Such bulging causes the molten steel remaining at the leading end of the crater and has been enriched with impurities to move toward the leading end of the crater so that the segregation caused by such remaining molten steel presents at the center of the solidified cast pieces. Such bulging also occures even when the roll pitch at the leading end of the crater is large thus causing segregation at the central portion. In each case, it is not desirable for the not yet solidified crater to come ahead of the pinch rollers and many efforts have been made for controlling the position of the leading end of the crater. However, until today, no effective method has been developed. More particularly, according to one method proposed, molten steel is continuously cast under a definite condition, radioactive substance or a substance not soluble in steel such as lead is incorporated into the molten steel before it is cast, so as to cause such substance to precipitate or diffuse to the leading end of the crater and then to solidify, whereby the position of the leading end of the crater can be determined by the position of such target substance. According to another method the position was determined by the calculation of heat conduction. Actually however it is difficult to accurately determine the position of the target substance due to the spacing between pinch rollers or other causes. According to a method disclosed in Japanese Patent Publication No. 40937 of 1972, there are provided mill rolls for reducing the thickness of the cast piece and a mechanism for detecting the rolling reaction. Where the rolling reaction varies rapidly the mechanism operates to control the quantity of the cooling water or the rolling speed. Although this method is effective for cast pieces having a relatively small cross-sectional area such as billets but accompanies a decided difficulty for cast pieces having larger cross-sectional areas such as slabs. More particularly, a slab having a thickness of 200 mm was continuously cast and immediately after its central portion has completely solidified the cast slab was rolled under pressure by mill rolls. Then the reduction rate and the rolling pressure were measured. The rolling pressure W (in ton) can be expressed by the following equation. EQU W (ton) = K.multidot.L.multidot.Y.multidot. (Y .ltoreq. 5%)
when K represents a coefficient which varies depending upon the type of the steel and the operating conditions and is generally equal to about from 0.18 to 0.27, L the width of the slab, and Y the reduction rate. From this it can be noted that where K = 0.23 a rolling pressure of 1720 tons is required to reduce 5% a slab having a width of 1500 mm. This means that an extremely large rolling pressure is required even for a small reduction rate. As a result it is necessary to install a mill having rolls of large stiffness and large diameter and a rigid housing. In a continuous casting apparatus, the roll pitch is generally 200-600 mm. from the secondary cooling zone to the pinch rollers. Larger pitch than this value causes the slab to bulge in the direction of thickness under the static pressure of the molten steel inside the crater. For this reason, it is impossible to install a roll housing that can withstand to the rolling pressure which amounts to 2000 tons or more. The roll housing that can be designed to meet said roll pitch requirement can withstand against a pressure of at most about 300 tons. It is clear that such roll can not be used practically.